Human hair has many textures, from fine to coarse and from straight to curly. Hair is made of keratin which in turn is made of polypeptide chains bonded together by cysteine (or disulfide) bonds, hydrogen bonds and salt linkages.
Curly hair is made of hair strands that have irregular surfaces that mesh and tangle with each other to make combing more difficult. Among individuals with very curly hair, e.g., individuals of African or Middle Eastern descent, it is especially popular to relax or straighten hair to increase hair manageability and ease of styling. Threads are composed of proteins of spiral structures, linked by a sulfur double-bond. These bonds are responsible for the hair structure and they may be broken by certain reduction reactions. The most common reductive agents are thioglycolic acid and its derivatives such as ammonium thioglycolate and bisulfite. These act on the hair keratin by breaking disulfide bonds that link cysteine units; this way, they form cysteine, the main component of keratin. The hair to be relaxed is exposed to a relaxer that chemically transforms cysteine bonds of the hair to lanthionine bonds. For this reason, the term for the chemical relaxing process is lanthionization.
Hair straightening or hair relaxing products have been commercially available for over forty years for people who want straighter and more manageable hair. Most commercially available hair relaxers are composed of a strong hydroxide base that breaks the bonds in the hair. The first straightening chemicals were developed around 1940. They were rudimental preparations of sodium or potassium hydroxide blended with starch and they were highly irritating to the scalp. By the 1950s, several kits for chemical straightening, based on sodium hydroxide, were introduced in the professional market. In 1965, the market already had a chemical straightening product for home use, based on sulfites. In 1985, Avon Industries introduced a straightening system based on types of hair and scalp sensitivity (i.e., normal and sensitive). The first one used sodium hydroxide as its active ingredient and the other used guanidine hydroxide. However, the results provided by existing straightening products are not optimal; nor is their safety. Current products present inconvenience issues, such as weakening of threads, irritation of the scalp and difficulty in rinsing. Also due to their instability, these formulations degrade rapidly.